The present invention relates to infrared detecting apparatus which is one of component elements of a moving human body sensor for sensing the infrared rays radiated from the human body, thus detecting the entry and the leaving of the moving human body into and from the supervisory area.
As generally shown in FIG. 11, such a moving human body sensor comprises an infrared detecting apparatus 1 and an optical system C disposed at the light incident side of the apparatus 1. In the infrared detecting apparatus 1, a shield case 5 having an opening 8 incorporates a circuit board 4 having an infrared detecting unit 2 and a variety of electronic elements 3 mounted thereon. The opening 8 is sealed by a transparent plate 9 to form a light receiving window. The circuit on the circuit board 4 is electrically connected to an external circuit through a cable 7 guided to the outside through a hole 6 in the shield case 5.
The infrared detecting unit 2 incorporates a pair of infrared detecting elements 12a, 12b having the same characteristics made of a pyroelectric material and differentially connected to each other, these elements 12a, 12b being disposed for converting incident infrared rays into electric signals. The infrared detecting unit 2 also incorporates a field-effect transistor 16 for amplifying signals supplied from the infrared detecting elements 12a, 12b. The infrared detecting elements 12a, 12b and the field-effect transistor 16 are covered with a case 17 having an optical filter 19 disposed at an infrared receiving opening 18, the optical filter 19 being adapted to transmit infrared rays only and to intercept visible light. The infrared detecting elements 12a, 12b have surface electrodes 15, conductive element support stands 14 and conductive adhesives 13. The optical filter 19 has its front surface coated with a reflectionless material, and its rear surface coated with multi-layer membranes as a band pass filter surface through which wavelengths of 6 to 14 .mu.m pass.
The optical system C comprises a convex lens group or a Fresnel lens group having different visual field groups, each being composed of, for example, five differently directed visual fields E1 to E5 or e1 to e5 as illustrated in FIG. 11. Incident infrared rays along two visual field groups formed by this lens group, pass through the light receiving window (the transparent plate 9) of the infrared detecting apparatus 1 and the optical filter 19 of the infrared detecting unit 2. Then, the infrared rays are condensed on the infrared detecting elements 12a, 12b.
The human body moving in a place sufficiently remote from the optical system C, successively passes visual fields belonging to the different visual field groups of the optical system C. Accordingly, detection signals of the infrared detecting elements 12a, 12b are supplied with certain time delays. On the other hand, there exists external disturbing light resulting from the irradiation of the sunlight or the headlights of travelling vehicles, or the radiation from the floor of the supervisory area. Such external disturbing light simultaneously covers visual fields belonging to the different visual field groups in a broad area. As a matter of fact, the infrared detecting elements 12a, 12b simultaneously receive infrared rays having the same intensity, and simultaneously supply signals in the same level. Accordingly, no signal is supplied from the infrared detecting unit 2 in which the infrared detecting elements 12a, 12b are differentially connected to each other. The visual portion of the external disturbing light is intercepted by the optical filter 19.
The moving human body sensor is originally adapted to detect the moving human body only under no influence of external disturbing light. However, there are instances where such sensor is erroneously operated due to external disturbing light such as the lighting of a patrol guard or the headlights of travelling vehicles.
Such an erroneous operation is caused by the fact that the moving human body sensor incorporates the optical system C. Since infrared rays are condensed on the infrared detecting elements 12a, 12b by the optical system C, the energy density of the infrared beam passing through the optical filter 19 is relatively high. Accordingly, the infrared rays are absorbed in the course of the transmission thereof through the optical filter 19. This causes the optical filter 19 to locally radiate heat, resulting in radiation of secondary infrared rays.
To arrange the entire infrared detecting unit 2 in a compact design, the optical filter 19 is disposed with a distance l as short as 0.5 to 1.5 mm provided between the optical filter 19 and the infrared detecting elements 12a, 12b. Accordingly, the spreading angles .theta.'.sub.1, .theta.'.sub.2 of light from a heat radiating part 19a of the optical filter 19 to the infrared detecting elements 12a, 12b are considerably different from each other, as shown by a chain line in FIG. 5. This produces a great difference between the amounts of secondary infrared rays received by the infrared detecting elements 12a, 12b. As a result, there is erroneously supplied a signal similar to a signal to be supplied when the moving human body is detected.
An infrared detecting apparatus having a single infrared detecting element, may also be erroneously operated due to the reaction of the infrared detecting element to the secondary heat radiation energy of the optical filter.
In the moving human body sensor, the infrared detecting unit 2 is electromagnetically shielded by the shield case 5. However, the shielding effect is insufficient since the transparent plate 9 serving as the light receiving window of the shield case 5 is not conductive. Accordingly, the infrared detecting unit 2 and circuit elements 3 mounted on the circuit board 4 cannot be perfectly protected from jamming.
Further, the shield case 5 for housing the infrared detecting unit 2 and the like is generally made of metal. Accordingly, if the environmental conditions such as the direct shining of the sunlight, the direction and speed of a wind, undergo a sudden change, the shield case 5 sensitively reacts to such a change, thereby to produce local variations of the case temperature even in a short period of time. This produces difference in intensity among the secondary infrared rays radiated from different parts of the shield case 5. This may cause the infrared detecting elements 12a, 12b to be erroneously operated.